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mq_notify(2)							  mq_notify(2)

NAME
       mq_notify - register/cancel a notification request with a message queue

SYNOPSIS
DESCRIPTION
       If the argument notification is not NULL, the system call registers the
       calling process to be notified of message arrival at an	empty  message
       queue  associated with the message queue descriptor mqdes.  The notifi‐
       cation specified by the notification  argument  will  be	 sent  to  the
       process	when the message queue transitions from the empty state to the
       non-empty state.

       If the type of notification specified in notification->sigev_notify  is
       then  the signal specified in notification->sigev_signo will be sent to
       the process.  If the flag is set for that signal number, then the value
       specified  in  notification->sigev_value will be the si_value component
       of the siginfo_t structure passed to the signal	catching  function  at
       the time of signal delivery.

       At any time, only one process can be registered for notification with a
       message queue.  If the  calling	process,  or  any  other  process  has
       already	registered  for notification with the specified message queue,
       subsequent attempts to register with that queue will fail.

       If notification is NULL and the process	is  currently  registered  for
       notification  with  the specified message queue, the existing registra‐
       tion is canceled.

       When the notification is sent to the registered process, its  registra‐
       tion is removed.	 The message queue is then available for registration.

       If there is some process blocked in waiting to receive a message from a
       message queue, the arrival of a message on the queue will  satisfy  the
       even if the queue has a registered notification request.	 The resulting
       behavior is as if the message queue remains empty, and no  notification
       is sent.

       To use this function, link in the realtime library by specifying on the
       compiler or linker command line.

RETURN VALUE
       returns the following values:

	      Successful completion.

	      Failure.
		   is set to indicate the error.

ERRORS
       If fails, is set to one of the following values:

	      [EAGAIN]	     The  system  lacks	 sufficient   signal   queuing
			     resources to honor the request.

	      [EBADF]	     The  mqdes	 argument is not a valid message queue
			     descriptor.

	      [EBUSY]	     A process is already registered for  notification
			     with the message queue.

	      [EINVAL]	     An	 attempt  was  made  to	 cancel a non-existent
			     notification request, or notification  points  to
			     an invalid address.

	      [ENOSYS]	     is not supported by the implementation.

SEE ALSO
       mq_open(2), mq_send(2), sigaction(2).  signal(2), signal(5).

STANDARDS CONFORMANCE
								  mq_notify(2)

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